Colbert



Feb. 14, 1956 s. GOEBERT 2,734,474

METHOD OF STRETCH-BENDING SHEET METAL Filed May 4, 1951 3 Sheets-Sheet 16 65 iv r.s

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Feb. 14, 1956 s. GOLBERT 2,734,414

METHOWOF STRETCH-BENDING SHEET METAL Filed May 4, 1951 3 Sheets-Sheet 2'llllliiu Feb. 14, 1956 s, GQLBERT 2,734,474

METHOD OF STRETCH-BENDING SHEET METAL Filed May 4, 1951 3 Sheets-Sheet 3JTJ/ Iwsurwg 36/! 6-6 azesr yw, d

United States Patent '0 i METHOD OF STRETCHeBENDING SHEET METAL SergeGolbert, Paris, France, assignor to Societe Nationale de ConstructionsAerouautiques du Sud-Est, Pans, France Application May 4, 1951, SerialNo. 224,545 7 Claims priority, application France May 15, 1950 2 Claims.01. 113-51 As a rule, the shaping of a limited number of convex sheetmetal parts subjected to a single or double curvature is performedthrough the conventional stretch-bending method in which the blank isheld along two generatrices by a pair of rectilinear side clamps andsubjected to the action of a shaping punch. The latter is fixed to themovable plate of a hydraulic press and moved towards and pressed againstthe blank.

The stretch-bending method is particularly suitable for the productionof generally rectangular sheet metal parts having a pronouncedtransverse curvature compared with its longitudinal curvature, which isthe case, for example, with fuselage elements. This method is alsoconvenient for forming elements having a single curvature;

During the stretch-bending operation the blank is subjected to arelatively low bending stress, just beyond the elastic limit, while thetensile stress applied thereto greatly exceeds this limit. With thisdistribution of the deforming stresses the elastic contraction is verysmall and the sheet practically retains the shape of the shaping die.However, this blank stretch-bending method otters a very seriousdrawback in that it leads to a substantial loss of sheet material. Infact, to ensure the progressive deviations of the lines of force betweenthe curved shape and the rectilinear jaws of the blank-holding clamps,more particularly for parts having a double curvature, requiressacrificing after stretch-bending a relatively large area of sheetmaterial in the zone which units the stamped part of the blank with therectilinear clamped parts or flanged edges thereof and to which thelength of sheet material clamped by the jaws and forming said flangededges is 'to be added. I

The aforesaid joining zone may be slightly reduced by using blankholders made of several elements arranged in polygonal fashion; however,this arrangement still requires the presence of a relatively widejoining zone to prevent, on the one hand, the formation of wrinkles dueto the compressive stresses exerted and, on the other hand, overstressedpoints where the jaws meet, both wrinkles and overstressed points beingincipient fractures as a rule. If the tensile stress is applied throughthe blank holders, the shaping die remaining still, the distribution ofstresses is improved but the conditions of operation and notably thedimensions of the wasted sheet material remain unchanged. a

In order to fully eliminate the aforesaid joining zone and thesubstantial loss in sheet material resulting therefrom, the main objectof the present invention is to provide a method of stretch-bending planethin sheet metal blanks wherein the blank freely bearing on a devicereproducing the contour of the longitudinal trim lines of the stampingto be obtained, is flexed bythe application along-a substantiallylongitudinal line thereof of a uniformly distributed stress less thanthe elastic limit of the material, forming said blank, in order toattain end heights with resp'ect to said contour at the mostequal tothose of the transverse trim lines of the stamping, longitudinal2,734,474 Patented Feb. 14, 1 956 being determined, then said thin sheetmetal blank is allowed to spring back to planar shape under its ownelasticity by removing said stress and in said planar shape ispre-formed with flanged edges used for clamping the same during thestretch-bending step, the inner edges of said flanged edges coincidingwith the developed drawn clamping lines while their angle with theremaining planar part of the developed blank is equal at each point oftheir inner edges to the angle determined at the corresponding point ofsaid clamping lines when the blank was flexed, then the flanged edges ofthe pre-formed blank are clamped by means of a clamping devicereproducing the contour of the longitudinal trim linesof the stamping tobe' obtained so that their inner edges coincide with said contour, whichinduces flexing of the blank part comprised between said flanged edges,owing to the fact that, on the one hand, the blank has been pre-formedwith flanged edges at a determined angle with respect to said part and,on the other hand, said inner edges in said clamped state are closertogether than the developed drawn clamping lines, said flexed part ofthe pre-formed and clamped blank being finally stretch-bent around ashaping die the shape of which reproduces that of the stamping to beobtained, the flanged edges being cut out after the stretch-bending stephas been performed.

With the novel method of this invention, it is possible to obtain agreater number of shapes than those afforded by rectilinear blankholders owing, on the one hand, to the better distribution of the linesof force and, on the other hand, to the reduction in the length of thestretched fibers, the allowable elongation being in reverse ratio to thelength.

The accompanying drawings forming part of this specification illustratediagrammatically by way of example a practical embodiment'of a devicefor carrying out the method of the invention. In the drawings:

Fig. l is a vertical section illustrating the fitting of a blank to beshaped by stretch-bending in holding jaws made in accordance with theteaching of this invention.

Fig. 2 is a perspective view of the bottom supporting members, withoutthe clamping or holding jaws, utilized for stretch-bending thin sheetmetal blanks, these members being disposed according to the contour ofthe longitudinal trim lines of the stamping to be obtained.

Fig. 3 is a perspective view showing the special frame used fordetermining and drawing the clamping lines on the sheet metal blank,said special frame being adjusted in accordance with a form reproducingthat of the stamping to be obtained.

Fig. 4- shows the position of a drawing bead supporting member in thespecial frame, in relation to the form.

I Fig. 5 is a central transverse section of the special frame shown inFig. 3 and illustrates the manner in which the blank used for obtainingthe stamping of Fig. 7 is set in position in the special frame afteradjusting the latter to the form.

Fig. 6 is a perspective view of the part of the blank flexed asillustrated in Fig. 5 which is comprised between the clamping linesdrawn on said blank, the frame and the means for flexing the blankhaving been omitted.

Fig. 7 is a perspective view of the same part of the blank after thestretching step giving the stamping to be obtained, the flanged edgeshaving been cut out.

Fig. 8 is a'diagrammatical illustration of the method of determining theangle made by the flanged edges with the blank flexed as illustrated inFig. 5 at a given point of theclamping lines drawn on the blank. 1 1Fig. 9 is a central transverse section of the jig used for forming theflanged or clamping edges of the blank on the developed blank used forobtaining the stamping of Fig. 7 with a flanged developed blank removedfrom the ig.

Fig. 10 is a diagrammatical central part section of the hydraulicdouble-acting press of a type suitable for stretch-bending metal sheetsor blanks, said press being equipped with a flanged blank submitted to asecond flexing by clamping its flanged edges on the clamping jawsillustrated in Fig. 2 and with a shaping die used for obtaining thestamping illustrated in Fig. 7.

Fig. 11 is a perspective view of the blank pre-forming jig illustratedin Fig. 9.

In view of stretch-bending a sheet metal blank according to thecharacteristic features of this invention, the continuous holding edgefor each longitudinal side of the blank consists of a clamping head 1,preferably of steel, having a diameter of 7 to 6" for example and curvedaccording to the contour of the shaping die which reproduces the contourof the longitudinal trim lines of the stamping to be obtained. Thiscurved clamping bead is held in position by a set of supporting members2, for example four inches wide, disposed on the bed plate of astretch-bending press and arranged according to said contour. Eachsupporting member is formed with a groove 3 in which the curved clampingbead 1 is laid, and a pair of deeper grooves 4, 5 receivingcorresponding ribbed rollers 6, 7 respectively. The blank 8, asexplained hereinafter with reference to Figs. 9 and 11, is formed withflanged edges 11 which are placed upon the curved clamping beads 1 androllers 6, 7, so that their inner edges coincide with said beads 1,whereby, as explained with reference to Fig. 10, the part of said blankdisposed between said flanged edges assumes a curved shape. In thisposition, the blank is clamped by means of upper jaws 9 through theintermediary of rubber strips 16 whereby the flanged edges 11 of blank 8will be pressed against rollers 6, 7. When the blank is thus positivelyheld against movement, its part contained between said flanged edges 11is shaped by lowering the shaping die 12 equipping the stretch-bendingpress and which acts in the direction of the arrow F.

As illustrated in Fig. 2 the supporting members 2 for the continuouscurved beads 1 are grouped in threes on intermediate supporting blocks13 displaceable and orientable on the bed plate of the press so that thecontinuous curved heads 1 are properly positioned on curvescorresponding to the contour of the longitudinal trim lines of thestamping to be obtained.

If sheet elements having a small transverse curvature are contemplated,that is to say elements in which the difference between arc and chord isof from 3% to 5%, a flat blank can be used initially for thestretch-bending operation, without prior formation of any flanged edge.

If the curvature is more pronounced, it is necessary to provide theblank with flanged edges for positively holding the same againstmovement by jaws 9 when the shaping die is moved in the direction ofarrow F. For this purpose, with a view to determine the clamping linesof said flanged edges 11 and to decrease the tensile stress which willbe applied to the blank when mounted on the press at a value less thanthe elastic limit of the metal forming said blank, according to thepresent invention, the blank lying on the curved beads 1 is submitted toflexing in order to attain end heights with respect to said heads at themost equal to those of the transverse trim lines of the stamping to beobtained, the clamping lines being drawn along said beads while theblank remains in its flexed state. In most instances and particularlywhen thin sheet metal is used, the free flexing of the sheet. under itsown weight obtained by laying the blank on the curved beads may besufficient to attain said end heights.

For mass production, instead of using the curved heads I mounted on thebed plate of the press, it is preferable to use a special frame in orderto avoid an undue stopping in the operation of the stamping device.

A practical example will be considered in which stampings having asymmetrical double convexity as illustrated in Fig. 7 are to beobtained.

Fig. 3 illustrates a special frame for drawing the clamping lines on theblank used for obtaining the stamping illustrated in Fig. 7. This deviceconsists of a rectangular bottom frame 16 having columns 17 connectedthrough transverse elements 18. On the bottom frame 16 is fixed alongitudinal beam 19 carrying lateral slideways 20. A pair of transversebeams 21, 22 contained in a common horizontal plane are carried in anadjustable or fixed manner by columns 17 and provided in turn withslideways 23, 24. The form 25 reproducing the shape of the stamping tobe obtained or of the shaping die is mounted in the special frame withthe assistance of a hoisting-conveying machine such as a travellingcraneor hoist and of a beam 26 which is fast with the form. In Fig. 3, thecurves which form the upper contour of form 25 and which correspond tothe longitudinal trim lines a'b'c and def of the stamping correspondingto form 25 are assumed to be contained in a common plane. With theassistance of legs 27 mounted for sliding vertical adjustment on theaforesaid slideways 20 and fast with a perpendicular element 28, on theone hand, and similar members 29 positioned at the rear of the frame onthe same slideways 20, the upper plane contour of the form is brought tothe horizontal position. Then, on each side of form 25, movable beams 30are mounted on transverse beams 21 and 22, said movable beams beingrespectively inclined in their common horizontal plane substantiallyparallel to the average inclination of the curves forming the uppercontour of form 25. On each movable beam 30 carrying a slideway 31 aseries of adjustable supporting members 32 are mounted. These membersare formed with an end groove in which a continuous drawing bead 33 islocated. The beads 33 are caused to take the shape of the curves formingthe upper contour of form 25, i. e. the shape of the longitudinal trimlines of the stamping, or at least to be placed in positions tangentwith the surface of this form, as illustrated in Fig. 6. The assemblyconsisting of members 27, 28, on the one hand, and members 29interconnected through a member similar to member 28,

on the other hand, defines the end heights or depths of form 25 inrelation to the place of its upper contour, i. e. the heights of thetransverse trim lines a'gd and c'if' of the stamping.

After the members 28 indicating the extreme or end heights of the formand supporting members 32 are adjusted, so as to properly position thecontinuous drawing beads 33, form 25 is removed from the special frameand replaced with a sheet or blank sunk under free flexing into theaperture provided between the drawing beads 33 down to the level of themembers 28 indicating the extreme or end heights. Then, this sheet metalblank is held in position in a state of constrained flexing by loadingit with a cylindrical bar 35 bearing on the pair of members 28 whichserve for determining the heights at both ends as shown in Fig. 5, whilesaid blank remains flexed under constraint. This causes the part of theblank comprised between the drawing beads 33 to assume the shape of acomposite regulated developable surface of the type illustrated in Fig.6.

When the blank is held in this position a marking tool is utilized formarking on the blank clamping lines corresponding to the position of thecontinuous marking beads 33. Then, with the assistance of specialdevices 36 formed With recesses for housing the drawing beads 33, asillustrated in Fig. 8, inserted on the continuous drawing beads 33between the supporting members 32 thereof and by applying the face 37 ofthese devices against the sheet material 34, horizontal lines,illustrated in dotted line on Fig. 8, are drawn on said devices 36.

At each point of said drawing beads 33 the anglea, which is the definiteangle at which the blank will be clamped in the machine jaws, isdetermined as the supplement of the angle existing between thehorizontal line drawn on the corresponding device 36 and the face 37 ofsaid device.

For more readily understanding the particular features of the invention,Fig. 6 represents the part of the blank, flexed as illustrated in Fig.5; which is comprised between the clamping lines drawn on said blank incorrespondance to the drawing beads 33. In this position, thesheet-material between the clamping lines is submitted only to flexingand has a regulated developable surface. The clamping lines abc and def,drawn on the blank and which are coincidents with the drawing beads 33and the shaping die contour, andthe depths pg and qi of said blank withrespect to said clamping lines, respectively coincide with thelongitudinal trim lines abc and d'e'f' of the stamping (Fig. 7) andwiththe heights p'g and qi' of its transversal trim lines a'g'b' and c'if'.On the contrary, the lengths of the intermediate longitudinal sectionallines in said part of the flexed blank, such as ghi (Fig. 6), areshorter than the corresponding longitudinal sectional lines of thestamping, such as ghi (Fig. 7), because the height or depth of said partof the flexed blank is the same throughout its length with respect tothe clamping lines, the line ghi being a straight line.

The longitudinal elongation is given by the ratio between the lengths ofcurve ghz" and straight line ghi.

The maximum transverse elongation is given by the ratio between therespective lengths of curves b'h'e' and bhe.

The related part of the flexed blank has seven developable sections: onecylindrical portion jklm, two flat triangles jbl and kem, and fourcylindrical elements ajb, bcl, mfe and ekd.

However, the accurate determination of these developable sections is notindispensable because the related part of the blank will take this formby itself when laid upon the drawing beads 33 (Fig. 5) which coincidewith the clamping lines and flexed by bar 35. The-clamping contour ofthe sheet can be determined by merely measuring depths or heights-pg andqi and determining lateral contours abs and def, which operation ashereinabove explained, may be carried out by utilizing either theclamping beads illustrated in Fig. 2 or the drawing beads illustrated inFig. 3.

After drawing the clamping lines on the flexed blank with a marking tooland measuring the angle on along said lines, the bar 35 is removed andthe blank is extracted from the drawing frame and is restored to adeveloped planar state by its own elasticity. From thisdeveloped blank ahard wood form of the kind illustrated in Figs. 9 and 11 is machined.This form consists of side members 38, the outer edges of which havebevelled portions 40 the inner edges of which reproduce the developedclamping lines drawn on the developed blank. Said bevelled portions 40make with the horizontal at each point of their inner edges 41 an angleequal to the angle a measured at the corresponding point of the clampinglines when the blank is placed on the drawing beads 33 in aflexed stateas illustrated in Fig. 5. Said side members 38 are interconnected bycross ties 39 the length of which is determined in order that" the inneredges 41 coincide with the developed clamping lines.

The blank in its developed planar state is mounted on said form so thatthe developed clamping lines drawn thereon coincide with the inner edges41. The edges of said blank are then bent on the bevelled portions 40 soas to flange the same along the developed clamping lines according tothe angles a measured at each point of said lines, whereby a flangedplanar blank is obtained.

As explained hereinabove, the flanged planar blank is mounted on astretch-bending press equipped with a shapingdie 12...;For this purposethe flanges are placed upon the curved clamping beads 1 (Figs. 1 and 10)so that their inner edges are substantially coincident with saidbeads 1. Under the action of the clamping jaws 9 the flanges and theinner edges occupy the positions illustrated in Fig. 1, said inner edgescoinciding with said clamping heads 1. Simultaneously, owing to the factthat, on the one hand, the blank has been pre-forrned with flanges atangles a with respect to the remaining part of said blank and, on theother hand, the clamping beads 1 are closer together than the developedclamping lines, a flexing of the part of the blank contained betweenthe'clamped flanges 11 is induced, as illustrated at 47 inFig. 10, saidpart corresponding to Fig. 5. The thus obtained form differs from thatillustrated in Fig.5 by the fact that, on the one hand, a constraint hasbeen applied during the flanging operationon the zone of the blankadjacent to the flanges and, on the other hand, any device like the bar35 is omitted, whereby the end heights of the form illustrated in Fig.10 are smaller than those of the form illustrated in Fig. 5. It is notnecessary to have accurately flattened-flanged edges, on the contrary,it is preferable to have them slightly cor: rugated so that a greaterflexibility willbe available when positioning the blank on thestretch-bending apparatus.

The stretch-bending device can be made either as a separate unit, or asaspecial attachment for a hydraulic press. When the hydraulic pressis ofthe double-acting type the fitting of the device thereon is facilitatedbecause the entire hydraulic equipment of the press is available. Atypical example of a mounting of this kind is illustrated in Fig. 10 ofthe drawings.

The clamping action exerted by jaws 9 which support clamping beads 1curved and disposed as drawing beads 33 may be assured through suitablepush-rods 42 so arranged as to register with each head supporting member2. These push-rods 42 are fixed beneath an intermediate plate 43 andadjustable thereon in the direction indicated. by the arrows f1 and f2.This intermediate plate 43 is controlled by the drivers 44 of theblankpressing plate of the machine. The convex bottom sur face ofshaping die 45 has substantially the shape of the stamping to beobtained and this shaping die is fixed to the ram 46 passing with a freeclearance through the intermediate plate 43. When driven in thedirection of the arrow F this shaping die will produce the permanentdeformation by stretch-bending of the clamped blank 8 which wasinitially clamped by the action of jaws 9 on its flanged edges so thatthe stamping illustrated in Fig. 4 is obtained after the flanged edgeshave been cut out.

However, the arrangement illustrated is purely diagrammatical and givenby way of example. Thus, the punch 45 may be driven from beneath, thepress may be of the single-acting type and in this case the push-rods 42are provided with resilient clamping means.

When utilizing the stretch-bending method forming the object of thepresentinvention, the loss of blank material, after the stretchingoperation has been performed, is very small when compared with the lossof blank material in the conventional stretch-bending methods. In fact,when the blank is flexed as illustrated in Fig. 5, the angles made bysaid blank with respect to the plane of the drawing beads 33 aresubstantially equal to the angles formed by the stamping with the planepassing through its longitudinal trim lines along said lines.Consequently, during the stretch-bending operation performed on thepress illustrated in Fig. 10, the part of blank close to the flangededges is submitted only to a small tensile stress which avoids theformation of wrinkles or overstressed points, while in the conventionalstretch-bending methods the part of the blank close to the flanged edgesis submitted to a tensile stress which greatly exceeds the elastic limitand which generates wrinkles and overstressed points. In theconventional stretch-bending method, when the blank has been shaped inaccordance with the stamping to be obtained, the part of said blankformed with wrinkles and overstressed points must be cut out whichcorresponds for example, as illustrated in Fig. 1, to the loss of theflanged edges and of a part of the permanently deformed blank. In thestretch-bending method according to the invention, on the contrary, thecorresponding portion of the permanent deformed blank is free ofwrinkles and overstressed points and, therefore, may form a part of thestamping. Consequently, the loss of sheet material is limited to theloss of the flanged edges themselves which may range from 1%" to 2%approximately for each flanged edge.

In case the construction of a separate machine is contemplated, thisproblem may be solved in various manners. It is also possible to carryout the conversion of an existing stamping machine having rectilinearclamping jaws into a machine having the characteristics set forth above.

Briefly, the successive steps for stretch-bending sheet metal accordingto the invention, in the case of a limited production of stamping, areas follows:

(a) Determination of the heights p'g'-qi of the transverse trim linesa'g'd'cif of the stamping and formation of curved drawing beads 33coincident with the longitudinal trim lines ab'c'a"ef of said stamping,by means of the frame illustrated in Fig. 3;

(b) Introduction of a metal sheet in said frame which metal sheet, whilefreely bearing on the curved drawing beads 33, is submitted to aconstrained flexing for attaining end heights pgqi at the most equal tothe determined heights p'gq'i, as illustrated in Fig. 5, then drawing onthe sheet which is maintained in said flexed state the position lines ofcurved drawing beads 33 which will be used as clamping lines anddetermination of angles on along said curved drawing beads 33;

(c) Extraction of the sheet from the frame, said sheet recovering aplanar shape by its own elasticity, and formation of side flanged edgesalong the thus developed drawn clamping lines by means of the formillustrated in Figs. 9 and 11 and which reproduces the angles a;

(d) Mounting and clamping of the thus flanged sheet on thestretch-bending device, the inner limits or clamping lines of theclamped flanged edges 11 coinciding with the holding or clamping beads 1curved and disposed as the drawing beads 33, i e. with lines reproducingthe longitudinal trim lines abc-d'ef' of the stamping to be obtained(Fig. 10);

(e) Stretch-bending of the metal sheet for obtaining under permanentdeformation the stamping (Fig. 7)

after the flanged edges 11 have been cut out.

For mass production, the first metal sheet only is submitted to allsteps (a) to (e). The following metal sheets are only submitted to steps(c), (d) and (e), so that the steps of preliminary determination (a) andclamping line drawing (b) are made only once for a large number ofidentical elements.

What I claim is:

1. A method of stretch-bending plane sheet metal blanks, moreparticularly to obtain double-curved stampings from thin sheet metal,which comprises laying freely the plane sheet metal blank on a contourwhich reproduces the contour of the longitudinal trim lines of thestamping to be obtained, flexing said sheet metal blank lying on saidcontour by the application along a substantially longitudinal linethereof of a uniformly distributed stress less than the elastic limit ofthe material forming the blank so as to attain end heights with respectto said contour at the most equal to those of the transverse trim linesof the stamping, drawing on said sheet metal blank maintained in flexedstate clamping lines exactly reproducing the curves forming saidcontour, determining the angle of said blank maintained in flexed statewith a horizontal transverse line at each point of said drawn clampinglines, removing said stress whereby said sheet metal blank springs backto planar shape by its own elasticity, forming along the developedclamping lines drawn on the plane metal sheet blank flanged edges usablefor clamping said blank during the stretch-bending step whereby theinner edges of said flanged edges coincide with the developed drawnclamping lines, the angle of said flanged edges with the remainingplanar part of the developed blank being equal at each point of theirinner edges to the angle determined at the corresponding point of saidclamping lines when the blank was flexed, clamping the flanged edges ofthe thus pre-formed blank so that their inner edges coincide with thecontour of the longitudinal trim lines of the stamping to be obtainedwhereby the blank part comprised between said clamped flanged edges isflexed, stretch-bending said part around a shaping die the shape ofwhich reproduces that of the stamping to be obtained, and cutting outthe flanged edges alone.

2. Method of stretch-bending plane sheet metal blanks, according toclaim 1, adapted for mass production of double-curved stampings from alot of plane sheet metal blanks, wherein, after drawing of the clampinglines on the first sheet metal blank of the lot maintained in flexedstate and determination of the angle of said first blank maintained inits flexed state with a transverse horizontal line at each point of saiddrawn clamping lines and after said first blank has sprung back to itsplanar state, each plane sheet metal blank of the lot is formed alonglines reproducing the developed clamping lines drawn on the first blankin planar state with flanged edges usable for clamping said blank duringthe stretch-bending step whereby the inner edges of said flanged edgescoincide with the developed drawn clamping lines, the angle of saidflanged edges with the remaining planar part of the blank being equal ateach point of their inner edges to the angle determined at thecorresponding point of the clamping lines drawn on said first blank whenthe latter was flexed, then the flanged edges of the thus pre-formedblank are clamped so that their inner edges coincide with the contour ofthe longitudinal trim lines of the stamping to be obtained whereby theblank part comprised between said clamped flanged edges is flexed,afterwards said part is stretch-bent around a shaping die the shape ofwhich reproduces that of the stamping to be obtained and finally theflanged edges alone are cut out.

References Cited in the file of this patent UNITED STATES PATENTS1,216,396 Avery et al. Feb. 20, 1917 2,119,630 Williams June 7, 19382,251,477 Wiseman Aug. 5, 1941 FOREIGN PATENTS 543,040 Great BritainFeb. 6, 1942

